Coated bearing surfaces

ABSTRACT

Improvment in the wear resistance of surfaces which bear and slide one on the other is achieved by coating at least one, and preferably both, of such surfaces with a base coating of a shear promoting alloy, for example, Cu-Ni-In or Cu-Co-In and then applying to both surfaces a low friction coating of a sulfide of an element from the group Mo, W and their mixtures.

United States Patent [191 Betts [4 1 Feb. 19, 1974 Related US.Application Data [63] Continuation-in-part of Ser. No. 86,078, Nov. 2,

1970, abandoned.

[52] US. Cl 252/12, 29/195, 308/241 [51] Int. Cl Cl0m 7/06 [58] Field ofSearch 252/12; 308/241, DIG. 9;

[56] References Cited UNITED STATES PATENTS 2,387,872 10/1945 Bell..308/D1G.9

3,075,279 1/1963 Haltner et a] 29/195 3,143,383 8/1964 Bamberger et a1.308/241 3,194,759 7/1965 Devine et a1 252/12 Primary Examiner-Daniel E.Wyman Assistant ExaminerJ. Vaughn Attorney, Agent, or Firm-Lee H. Sachs;Derek P. Lawrence [57] ABSTRACT lmprovment in the wear resistance ofsurfaces which bear and slide one on the other is achieved by coating atleast one, and preferably both, of such surfaces with a base coating ofa shear promoting alloy, for example, CuNiln or CuCo-ln and thenapplying to both surfaces a low friction coating of a' sulfide of anelement from the group Mo, W and their mixtures.

5 Claims, No Drawings COATED BEARING SURFACES This application is acontinuation-in-part of application, Ser. No. 86,078, filed Nov. 2, 1970and now abandoned.

BACKGROUND OF THE INVENTION In general, wear can be described as theprogressive loss of material from rubbing surfaces. The rate and effectof such removal depends on many factors relating to properties of thematerial at the rubbing surfaces, environmental conditions and thetolerance of the mechanical device including such surfaces to acceptmaterial loss from its component. Because wear can lead to componentfatigue failure induced by failure of contacting surfaces, wear at theinterface between assembled jet engine components particularly thoserotating at high speeds, draws significant attention.

One approach for preventing wear at the interface between suchcomponents is described in US. Pat. No. 3,143,383 Bamberger et al. Thatpatent is directed particularly to wear improvement between contactingsurfaces of iron, nickel, cobalt, the refractory metals or their alloys.However, with the advancement of jet engine and metallurgicaltechnology, titanium alloys have been introduced into certain jet enginecomponents. Now, not only are there high performance requirements forthe superalloys and refractory metal alloys but also the specialproblems associated with titanium wear have been introduced.

SUMMARY OF THE INVENTION It is a principal object of the presentinvention to provide an improved coating system on opposed articlesurfaces bearing on one another which system protects the articlesurface from wear and consequent loss of fatigue life while at the sametime allows low friction movement between the surfaces.

Another object is to provide for titanium or titanium alloys such acoating system which is a composite, on at least one of such surfaces,of a shear promoting base coating and a low friction coating on both ofsuch surfaces, so that the low friction coatings contact one another andthe base coating on at least one of the surfaces beneath the lowfriction coating absorbs effects of relative motion between thesurfaces.

These and other objects and advantages will be more fully understoodfrom the following detailed description of the preferred embodimentswhich are meant to be typical of, rather than limiting on, the scope ofthe invention.

Briefly, the present invention provides, in a plurality of members incontact one with another at opposed bearing portions, the combination ofa base coating bonded to at least one of, and preferably both of, suchbearing portions and comprising a shear promoting alloy of Cu, anelement selected from Co and Ni and including the metallic element In,the oxide of which provides internal lubrication characteristics; and alow friction coating applied as a top coating over each of the bearingportions. The low friction coating includes a sulfide of the chemicallyrelated elements of the group Mo, W and their mixtures. The members bearone on the other through the low friction coating.

DESCRIPTION OF THE PREFERRED EMBODIMENT The fatigue life of titaniumarticles has been known to be seriously reduced when in contact with oneanother under high pressure sliding wear conditions without adequateprotection. For example, such action can occur at the juncture betweenblade dovetails and wheel slots of jet engine compressors. Though somecoatings protect such articles for relatively short periods of time,once wear-through occurs, article fatigue life is reduced rapidly at thearea of wear.

One coating material presently used to inhibit such wear on one of aplurality of such bearing surfaces is a Cu-Ni base alloy which includesIn to function, at least in part, as an alloy internal lubricant. In oneform, the material includes, by weight, about 20 40% Ni, about 2 10% In,with the balance essentially Cu.

As will be seen in the wear test data presented later, such a coatingalone provided inadequate protection when applied only to one surface ofbearing Ti or Ti alloys. Surface damage from wear resulted in a 25 30percent loss of alternating stress capability. Furthermore, such acoating applied to both surfaces in mutual contact was also inadequate,resulting in severe adhesive wear because of the mutually soluble,similar metals.

In an evaluation of the galling resistance of coatings under highcrush-stress and displacement of the type found in the fan portion ofadvanced turbofan engines, a forged titanium base alloy includingnominally 6% Al and 4% V was tested.

This evaluation employed reduced section specimens rubbed at roomtemperature and 50,000 psi contact pressure by reciprocal motion betweena specimen and a pressure wear shoe. After rubbing for the selectednumber of cycles, the specimen was tested for fatigue strength todetermine any loss. The specimens were 2 inches X 6 inches X V4 inchflat reduced section plates which were low stress ground and shot peenedprior to any coating. As shown by the data of the following Table I,none of the samples in the various combinations tested with a variety ofcoating materials on either or both the pressure wear shoe and thespecimen adequately protected the specimen from wear.

TABLE I Coating Example Shoe Specimen Thickness (mils) Galling l0 CyclesWear Remarks 1 none none l0 severe, metal flaking 2 none Cu 35 Ni 5 In 725 shoe severe, coating damaged 3 none Ni 25 graphite 6 24 shoe severe,coating damaged 4 none 75 Ni 25 Ag 6 l3 shoe severe, coating damaged 5none Graphite 2 3 severe shoe and specimen 6 none I, grease 6 severeshoe and specimen 7 none Polyurethane 7 7 moderate, coating removed 8none WC-l2Co+Mo S 3 5 'shoe severe 9 none PTFE(a) 3 l0 severe, coatingworn l0 Ni 20 Ag 80 Ni 20 Ag 6 4 severe, coating flaked TABLE I 'comiimfCoating Example Shoe Specimen Thickness (mils) Calling l" Cycles WearRemarks 1 1 Cu 35 Ni ln Cu 35 Ni 5 ln 6 25 localized severe l2 PTFE (a)PTFE (a) 3 25 localized severe 13 60 Cu 40 Ni 60 Cu 40 Ni 3 l8 coatingrough, galled l cnd l4 Cu 35 Ni 5 ln WCJZCo-l-Mo S 2 38 1 end; WC flakedcoatmg rough (an polylelral'luumethylene A major wear situation whichconfronts the use of The coating of Cu-35% Ni-5% 1n, unlubricated withsuch titanium base alloys in a high mass component the low friction Mo Scoating, when rubbed against unsuch as the fan of a turbo fan engine isthe wear of the coated titanium alloy, represented by Examples 16 anddovetail pressure faces as the fan blades seat and unseat 19, resultedin a friction coefficient of about 0.6. By in response to engine speed.Specifically, hoop stresses comparison, the specimens and shoes eachcoated with cause the wheel to grow circumferentially and the a basecoat of the Cu-Ni-In alloy and then coated with dovetail slots to expandand later contract thereby althe low friction Mo S coating, representedby Examlowing the dovetail to slide with the relative motion of ples 17,and 21, had friction coefficients on only about 0.005 inch across thepressure faces. This occurs 20 0.02 0.04 through 10,000 cycles ofrubbing. The generally twice during a flight: once during take off andspecimens, represented by Examples 15 and 18, subonce during thrustreverse upon landing. Therefore, an jected to wear with no coating allexhibited galling with additional series of tests was conducted toevaluate faseverity increasing between 1,000 and 5,000 wear cytigueeffects on samples subjected to increments of cles. Photomicrographicstudies indicated surface prior wear up to 10,000 cycles. Samples to becoated cracks which emanate from the galling damaged area were firstgrit blasted then flame sprayed with a Cuon all such wear specimens.Accordingly, the samples N i-5% In alloy after which a low frictioncoating represented by Examples 15 and 18, as well as those of thedichalcogenide Mo S was applied to selected represented by Examples 16and 19, including a coatsamples. When coated, the specimen and matingwear ing of the Cu-Ni-ln alloy only on specimens rather than shoes wereprepared in the same way. Samples were 3 on the shoe and with no lowfriction coating, showed rubbed under a pressure of 50,000 psi with a0.006 inch very early fatigue failure.

stroke at 60 strokes per minute. Coated samples with Mo S lubricant overthe Cu- TABLE 11 Avg. Test Data preworn samples Coating thicknessnominally 0.006 inch Example Coating Prior Wear Fric. Coef. Wear surfaceappearance Bonding Stress l0 Cycles to Failure Cycles 10" psi 15 none1000 0.62 moderately scuffed 35 65 16 1A 1000 0.58 coat flaked; spec.scuffed 45 39 I7 2AB l000 0.02 lightly striated 45 4700 18 none 50000.68 moderately galled 34 19 1A 5000 0.59 coat flaked;spec. galled 21 20ZAB 5000 0.04 lightly striated 45 6500 21 ZAB 10,000 0.04 lightlystriated 45 6090 key: I=Spccimen only: 2=Shoe and Specimen; A=Cu 35 Ni 5ln base coat; B=Mo S top coat The wear surface appearance in Table 11describes Ni-ln coating, by contrast, exhibited only uniform thespecimens as striated," scuffed or galled. lightly striated wearsurfaceswith no visible evidence These are terms which relate to increasingseverity of of substrate damage regardless of the exposure 'to atapparent superficial wear damage. Striated specimens least 10,000 wearcycles. When such titanium surfaces appeared to be the least worn,having only shallow 5 were examined after the coatings were chemicallyrestreaking of the surfaces toindicate relative motion. 'moved, theywere found to be completely protected Scuffing gives the impression ofslight metal loss in adfrom damage through at least 10,000 wear cycles.

dition to wear striations. Calling may be described as When no Mo 8 wasapplied to the base coating, the a form of wear resulting in securing,tearing and transcoating seized, adhered to the non-lubricated coatedfer of surface material under alternating motion. The shoe and wasstripped from the specimen in fewer than basic wear mechanism, adhesion,is clearly active in 10,000 cycles. Significant wear of the titaniumalloy producing the effect described although the other surface wasobserved after stripping of the coating from mechanisms, abrasion,corrosion, and surface fatigue such samples. This accounts for thefatigue loss which are involved to a lesser extent. was as severe aswith the intentionally uncoated tita- Friction data in Table 11 clearlyshow the lubricating 6Q nium samples.

benefit of the dichalcogenide Mo 8;. The uncoated The high cycle fatiguetesting reported in Table 11 samples, represented by Examples 15 and 18exhibited was conducted under 40,000 psi mean tensile stress an averagecoefiicient of friction of 0.65 with the averwith alternating bendingstress to failure. These data age for 1,000 cycles of 0.62 and for 5,000cycles 0.68. revealed that the grit blasting and subsequent coatingBecause of the galling and scuffing on these uncoated 5 did not affectthe base line fatigue strength of the alloy samples, no uncoated sampleswere further tested up to under the test conditions. Also, it was foundthat the 10,000 cycles. combination of a first coating bonded with bothof the bearing surfaces of a Cu-Ni-In alloy covered with the lowfriction dichalcogenide, represented by Mo S2, afforded complete surfaceprotection for at least 10,000 cycles prior wear based on fatiguestrength preservation. Also revealed was the fact that wear damage totitanium surfaces rubbed against titanium for nonlubricated base coatsof the type tested severely reduced the high cycle endurance capability.Thus the combination tested, which represents the preferred form of thepresent invention, provides a very effective wear protection system. Itforms an adequate bond with the titanium substrate to prevent anyrubbing of the titanium surfaces. In addition, it appears to provide inthe base coating of a shear promoting alloy" a relatively soft,lamellar-structured intermediate layer tending to equalize contactstress across the pressure face. Furthermore, such base coating also mayyield internally under low amplitude cyclic shear stresses to minimizerelative motion at the coating surfaces. The presence of a lubricativeoxide provided by the lower melting In appears to contribute to thesecharacteristics.

In still another series of tests, additional base coating materials aswell as other secondary or low friction type coatings were evaluated. Inthese tests, the base coating was applied to no more than one of thetworubbing components and the same low friction coating was applied toboth rubbing components so that the two components rubbed one on theother through the low friction coating.

juncture between fan dovetails and wheels in a fan jet engine, not onlywas the coefficient of friction rela tively high, as shown by Example25, but also there was severe galling of the wear surfaces. In addition,testing of the specimen after wear cycling showed a drastic reduction inspecimen fatigue life.

Frequently, the friction coefiicient between rubbing surfaces is used asa measure of the value of coating or coating systems to protectunderlying article surfaces from wear and consequent loss of fatiguestrength. However, it has now been recognized that such data cansometimes be misleading and that actual fatigue life data must be usedto fully evaluate such coatings or systems.

Another series of tests was conducted to evaluate the equivalency ofsulfide of tungsten to sulfide of molybdenum at various temperaturesunder rubbing conditions. Shoe specimens of the titanium base alloyincluding nominally 6% Al and 4% V were first coated with a Cu-Co-Inalloy base coat in the range, by weight, of 20 40% Co, 2 10% In with thebalance essentially Cu. More specifically in this series, thecomposition was nominally Co, 5% In, balance Cu. Application to the shoespecimens was by flame spray.

Low friction coatings were then applied over the base coating in variousgroupings of specimens, a M05 coating over one group and a WS coatingover another. The method of applying the W5 was by plasma spray whichdeposited a coating of 0.005 inch nominal TABLE Ill Bending Stress:45,000 psi Coating thickness nominally 0.006 inch Average FrictionCoefficient Coating l 5000 cyc. 10,000 cyc.

Spec.

Shoe Wear Surface Avg. l0 Cycles to Appearance Failure 22 No No NoneNone l 1 I6 23 0.25 0.3 B B galled 59 24 0.01 0.01 8* AB shallow spots3500 25 0.23 C AC severe galling 166 26 0.02 8* DB few galled spots I8827 0.18 B" EB severe galling 105 Vapor honed to hold coating l) key:A=Cu-35 Ni-S In base coat; B=Mo S, top coat; C=graphite top coat; D=60Cu-40 Ni base coat; and E=Ni-l8 Al base coat.

' Although the double-surface combination Cu-35% Ni-5% In/Mo 52 system,represented by Examples 17, 20 and 21 of Table II, is the preferred formof the present invention because of the full protection it affords thebase surface, the summarized data of Table III show the benefit ofanother form of the present invention for certain applications as shownby Example 24. In that Example, the rubbing of Mo S coated surfaces,with a single base coat of the Cu-Ni-In alloy on the shoe, protected thespecimen for up to 10,000 cycles as shown by the fatigue life. Ofsignificant interest in connection with this series of tests is the factthat, as shown by Example 26, a low coefficient of friction does notnecessarily mean that there is or will be no damage to the underlyingbase metal. In Example 26, there is a significant reduction in fatiguelife of the specimen even though the friction coefficient is very low.

This series of tests, represented by the data of Table III, comparedthree base coatings along with the low friction coatings of thedichalcogenide type represented by Mo S and the widely used solid filmlubricant graphite. It should be noted that under the conditions of thistesting, simulating those experienced at the thickness.

an opposed stationary block in 400F tests at 400 cycles per second under1,000 psi with 0.005 inch double amplitude movement for 20 hours tosimulate sheet metal rubbing wear conditions.

The conclusion from these tests was that the W8 coating was equivalentto or better than M08 coatings at 400F, although W8 did not perform aswell in other tests at 650F.

The present invention recognizes that protection can be afforded rubbingsurfaces through the application to at least one of the surfaces, andmuch more preferably to both of the surfaces, of a copper-nickel or acoppercobalt base alloy including an alloying element which functions asdoes In, each rubbing surface finally being covered with a sulfideselected from Mo, W and their mixtures. In this way, the sulfidesurfaces rub one on the other and the article surface is cushioned fromrelative motion damage through a shear promoting Cu Ni or Cu Co basealloy.

What is claimed is:

1. In a plurality of members in contact one with another at opposedbearing portions, the combination of: a base coating bonded with atleast one of the bearing portions and comprising an alloy of Cu, In andan element selected from the group consisting of Co and Ni; and a lowfriction coating applied as a top coating over each of the bearingportions, the low friction coating including a sulfide of an elementselected from the group consisting of molybdenum, tungsten and theirmixtures, the members bearing one on the other through the low frictioncoating. 2. The combination of claim 1 in which the base coating alloyconsists essentially of, by weight, about 20 40 percent of an elementselected from the group consisting of Co and Ni, about 2 10% In, withthe balance Cu.

3. The combination of claim 2 in which the base coating alloy consistsessentially of, by weight, about 20 40% Ni, about 2 10% In, with thebalance Cu.

4. The combination of claim 3 in which the sulfide is M08 5. Thecombination of claim 4 in which the sulfide is W8

2. The combination of claim 1 in which the base coating alloy consistsessentially of, by weight, about 20 - 40 percent of an element selectedfrom the group consisting of Co and Ni, about 2 - 10% In, with thebalance Cu.
 3. The combination of claim 2 in which the base coatingalloy consists essentially of, by weight, about 20 - 40% Ni, about 2-10% In, with the balance Cu.
 4. The combination of claim 3 in which thesulfide is MoS2.
 5. The combination of claim 4 in which the sulfide isWS2.